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The third manned mission of NASA’s Mercury program was Mercury 6, or Mercury-Atlas 6. To clarify the numbering system, Mercury 1 was unmanned and failed, but Mercury 1A, also unmanned was a success, followed by a successful Mercury 2 with Ham the chimpanzee on board, who survived the flight. Mercury 3 and Mercury 4 were the first manned suborbital flights for the US, and were successful. Mercury 5 was another chimpanzee flight, this time with Enos making the flight in the first use of the Atlas rocket for the Mercury program.
Manned flights resumed for Mercury 6, and this time the US would at last send an astronaut into Earth orbit, to match what the USSR had already accomplished. John Glenn was the astronaut on board, who named his capsule Friendship 7. After almost a month of delays due to weather and various technical issues including a fuel tank leak and a broken bolt on the hatch, Glenn launched on February 20, 1962 at 9:47:39 am EST from Cape Canaveral. He orbited Earth three times, with his flight lasting 4 hours and 55 minutes. In total he flew 121,793 km (75,679 miles) reaching a speed of 28,234 km/h (17,544 mph.)
The objectives for the flight were to place a human into Earth orbit, observe his reactions to the space environment and safely return him to Earth. Also, NASA wanted to make sure they could maintain optimum spacecraft attitude for radar tracking and communication checks.
The total time Glenn was weightless was 4 hours 48min. He ate food from tubes, like toothpaste tubes, to see how astronauts responded to eating in space, and he reported he felt fine. During the flight two major problems were encountered: a yaw attitude control jet apparently clogged at the end of the first orbit, forcing Glenn to abandon the automatic control system for the manual-electrical fly-by-wire system; and also a faulty switch in the heat shield circuit indicated that the clamp holding the shield had been prematurely released, a signal later found to be false. During reentry, however, the retropack was not jettisoned but retained as a safety measure to hold the heat shield in place in the event it had loosened.
Prior to the flight there had been concerns regarding the physiological effects of prolonged weightlessness and exposure to radiation on the astronauts. Glenn reported that the zero g conditions were “very handy” in performing his tasks and that he felt exhilarated during his 4.5 hour weightless period. It was later ascertained by physicians that Glenn had also received less than half of the expected radiation dosage during his flight, proving that the spacecraft walls had provided excellent shielding.
There was a bit of mystery during the flight, as Glenn saw what he called “fireflies” outside his capsule. Glenn reported “I’ll try to describe what I’m in here, I’m in a big mass of small particles, a whole shower of them coming by and they are brilliantly lighted. They average 7 or 8 feet apart, and I can see them all around me, and down below me also.” Ground controllers were concerned. A few minutes later Glenn said, “Just as I looked back out the window, I had literally thousands of small luminous particles swirling around the capsule and going away from me at maybe 3-5 miles per hour. Now that I am out in the bright sun, they seem to have disappeared.” What were the fireflies? The answer wasn’t confirmed until the next flight by Scott Carpenter: they were pieces of frost that came loose from the outside of the capsule. Frost formed from condensation on the spacecraft, and was illuminated by sunlight.
Glenn’s capsule splashed down in the Atlantic ocean 800 miles southeast of Bermuda and was recovered by the destroyer USS Noa 21 minutes later. Glenn remained inside Friendship 7 during the recovery.